1. Field of the Invention
The present invention relates to a balancer device for canceling a vibromotive force of a four-cycle V-type eight cylinder engine, and further relates to an oil return passage for an engine and more particularly to a passage for returning oil from a cylinder head to an oil pan.
2. Description of the Related Art
When a plane crank in which centers of all crank pin shafts are placed on the same plane is adopted in a four-cycle V-type eight cylinder engine having an included bank angle of 90 degrees, both banks operate at a cycle of 180 degrees and intake and exhaust strokes happen in an alternate fashion. According to this construction, intervals between intake and exhaust timings become equal and there is no risk of generation of exhaust interference, this being thereby advantageous in obtaining a high output.
On the contrary, in the case of a V-type eight cylinder engine adopting the plane crank, a secondary imbalance is generated by virtue of the inertia force of a reciprocating mass, and the direction of the inertia force when the engine is viewed as a whole when this happens is similar to a condition in which cylinders of a in-line four cylinder engine are placed horizontally (a horizontal component of the inertia force remains). Therefore, this imbalance can be compensated for by adopting the secondary balancer theory of the conventional in-line four cylinder engine and rotating in opposite directions two balancer shafts disposed on a plane in parallel with a plane bisecting the bank included angle of the engine and passing through a center of a crankshaft thereof (Japanese Unexamined Patent Publication No. HEI 8-193643).
However, in trying to provide balancer shafts on a V-type engine, it is inevitable that the construction of the engine becomes complicated, and since the balancer shaft itself generates an imbalance, a high degree of supporting rigidity is required in order to obtain an expected vibration damping function. Namely, provision of balancer shafts inevitably causes an enlargement of the engine and reduces the productivity thereof.
Further, conventionally, oil returning from a cylinder head is constructed so as to flow into a crank chamber through a passage provided in the cylinder block to be allowed to eventually fall in an oil pan defined below the crank chamber (refer to the Japanese Unexamined Patent Publication No. HEI 2-245413).
On the other hand, the Japanese Unexamined Utility Model Publication No. SHO 64-34449 discloses an engine comprising a cylinder block comprising in turn an upper block, a lower block and an oil pan adapted to be joined to a lower surface of the lower block, the upper and lower blocks being divided from a horizontal plane passing through a center of a crankshaft, wherein a baffle is formed integrally with the lower block for suppressing the generation of a disturbance in the surface of oil in the oil pan by virtue of a wind pressure generated by counterweights provided on the crankshaft or the movement of the oil surface by virtue of a change in the vehicle posture.
According to the aforesaid conventional constructions, since return oil interferes with a balancer shaft or the crank chamber and the oil pan are partitioned by the baffle, oil returning from a cylinder head is prevented from quickly returning into the oil pan, and the atomization of oil is promoted since return oil interferes with the balancer shaft and/or oil in the oil pan is easy to be taken up by the counterweights. This leads to an increase in the volume of oil intruding into a ventilation passage of the crank chamber, and if it gets worse, such atomization of oil may lead to a need to increase the capacity of an oil separator chamber.